1. Field of the Invention
This invention pertains to underground boring machines and their method of use. More particularly, this invention pertains to an underground boring machine with an improved mechanism for driving a drill head.
2. Description of the Prior Art
In the prior art, there are many examples of directional boring machines, one of which is shown in U.S. Pat. No. 4,953,638. In the '638 patent, an underground boring machine includes a frame (element 20) on which is mounted a drive mechanism (element 24) which can be slidably moved along the longitudinal axis of the frame. The drive mechanism is coupled to a drill string (element 42). The drive mechanism can rotate the drill string about its longitudinal axis. Further, sliding movement of the drive mechanism along the frame causes the drill string to be longitudinally advanced into or withdrawn from the ground. The '638 patent illustrates the use of a chain drive assembly (element 36) for moving the drive mechanism along the frame.
In many prior art designs, a single hydraulic cylinder provides the motive power for moving a chain. Due to the entrainment of the chain around pulleys, an incremental stroke of the cylinder piston would result in a displacement of the drive mechanism equal to about twice the stroke of the piston. However, this would result in the force of the drive mechanism equaling about half of the force applied by the cylinder. Also, any given movement is limited by a cylinder stroke.
When advancing a drill string through the ground, a small-diameter drill string is typically advanced through the ground such that the leading end of the drill string eventually is exposed at a remote location from the drilling apparatus. A reamer device is then attached to the end of the drill string and the drill string is pulled back through the bore initially formed by the boring apparatus. The purpose of the reamer is to increase the size of the bore formed by the drilling machine.
In order to accommodate drilling through adverse conditions (such as hard rock) and to accommodate the formation of large bores on pullback, extremely powerful underground boring machines have been developed. For example, underground boring machines have been developed with maximum thrust or pullback forces of 500,000 pounds (2,224 kN). In such large machines, additional means for advancing the drilling drive mechanism have been developed. These include direct rack and pinion drive systems to replace chain drive systems such as those shown in U.S. Pat. No. 4,953,638. However, in order to withstand the enormous forces associated with such large machines, the rack and pinion direct drive systems are massive in both size and weight.
It is an object of the present invention to provide a mechanism and method for advancing the drive mechanism of an underground boring machine with reduced size and weight when compared to prior art direct drive rack and pinion systems and with capability for handling high-force directional drilling machines (for examples, in excess of 250,000 pounds (1,112 kN)).